Optical waveguide fibers have been greatly improved during the last decade. Fibers exhibiting very low losses are generally formed by chemical vapor deposition (CVD) techniques which result in the formation of extremely pure materials. In accordance with these techniques, optical waveguide preforms can be formed by depositing and thereafter consolidating glass particles called "soot" on the surface of a mandrel by outside vapor deposition (OVD), or on the inside surface of a tube which later forms at least a portion of the cladding material, or by some combination of these techniques.
Although CVD techniques of forming optical waveguide preforms result in the formation of optical waveguide fibers have extremely low attenuation, such techniques are relatively expensive. Fiber manufacturing cost can be lowered by increasing preform size or by continuously drawing fiber from a preform while the preform is being formed. Both of these cost reducing techniques decrease the number of preform handling and processing steps per unit of fiber length.
The OVD technique readily lends itself to cost reduction modifications. Initially, preforms were made larger by increasing the diameter. This was accomplished by traversing the burner longitudinally along the soot preform and adding thereto additional layers of increasing radius. Thereafter, axial techniques were developed whereby one or more burners or other soot depositing nozzles were directed axially toward a starting member. As the thickness of the deposited soot layer increases, the starting member moves away from the burners. Axial vapor phase oxidation techniques are taught in U.S. Pat. Nos. 3,966,446, 4,017,288, 4,135,901, 4,224,046 and 4,231,774.
A hybrid technique whereby a core is formed by axial vapor phase oxidation, and a cladding layer is simultaneously deposited on the core by radially inwardly directed glass soot streams is taught in U.S. Pat. Nos. 3,957,474, 4,062,665, and 4,310,339. As the core is formed, it is withdrawn from the burners or nozzles which formed it. The cladding is deposited by stationary burners or nozzles.
Substantially continuous methods of forming optical waveguide fibers by vapor phase oxidation techniques are taught in U.S. Pat. No. 4,230,472 issued to P. C. Schultz and U.K. Patent Application No. GB 2,023,127A.
Deposition rate in the aforementioned CVD processes is determined by the temperature gradient within the gas stream in which the soot particles are entrained. See the publication, P. G. Simkins et al., "Thermophoresis: The Mass Transfer Mechanism in Modified Chemical Vapor Deposition", Journal of Applied Physics, Vol. 50, No. 9, September, 1979, pp. 5676-5681. Thermophoresis drives the soot particles from the hotter parts of the gas stream toward the cooler parts. Because the substrate or preform surface is usually cooler than the surrounding gas stream, the action of thermophoresis tends to drive the soot particles toward the preform surface. When a surface is nearly as hot as the surrounding gas stream, the temperature gradient is low. Thus, the thermophoresis effect is minimal, and the deposition rate is low. However, when the surface temperature of the preform surface is low, the thermophoresis effect due to the large thermal gradient results in a relatively high deposition rate.
In the aforementioned hybrid technique, a burner is continuously directed at one position on the preform. Thus, the preform surface becomes hot, and the rate of deposition is limited by the small temperature gradient between the preform surface and the soot-containing gas stream.
The method described in my U.S. Pat. No. 4,378,985 achieves an enhancement in deposition rate and efficiency by advantageously utilizing the thermophoresis effect. In accordance with that method a porous glass optical waveguide preform is formed by depositing a coating of glass particulate material on the lateral surface of a core which may be a porous glass body produced by the axial deposition of glass particulate material. The core rotates and moves longitudinally in one direction with respect to a flame hydrolysis burner. In addition, the burner reciprocatingly moves with respect to a portion of the length of the core. The reciprocating motion of the burner relative to the core material permits the soot preform to cool down between successive burner passes, thus increasing deposition rate due to enhanced thermophoresis. The continual longitudinal movement between the burner and the core material permits the formation of relatively long preforms or the continuous production of preforms from which fibers may be continuously drawn, if so desired. To form a radial gradient in the refractive index of the preform, the composition of the reactant vapor fed to the burner is varied in accordance with the position of the burner along its reciprocating path. During the reciprocating movement of the burner a conical section of preform is formed.